The Spotify backend consists of multiple microservices that communicate with each other. For microservices to be able to find each other, we utilize multiple service discovery technologies. Most of our services are using a DNS based service discovery system; however, some of our services use an xDS based traffic control plane and discovery system called Traffic Director.
On March 8, Google Cloud Traffic Director experienced an outage. This in coordination with a bug in a client (gRPC) library caused the Spotify outage that affected many of our users: if you were logged out of a Spotify app, you were unable to log back in.